1. Field of the Invention
The present invention relates to a device for reducing electrostatic discharge (ESD) damage in thin film read heads which enables measurement of gap resistances and, more particularly, to such a device and method wherein the resistance of first and second gap layers can be measured in parallel or the resistance of each of the first and second gap layers can be measured separately.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance GMR read head employs a spin valve sensor for sensing the magnetic field signals from the rotating magnetic disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The sensor and the first and second leads are located between first and second dielectric read gap layers which are, in turn, located between ferromagnetic first and second shield layers. Accordingly, the GMR head is electrically isolated from the two shields by the first and second gap layers which are typically aluminum oxide (Al2O3). The gap length, which is the distance between the shield layers, is continually being shortened in order to achieve higher areal density. For a given sensor thickness, therefore, the gap layers have to become thinner. In head designs, the shields are typically not electrically connected to any other conductors on the slider, and are electrically isolated from each other. As a result, a charge may accumulate on the shields during processing. The presence of this charge causes a potential difference across the gap layers. When this voltage reaches a sufficiently high value, the dielectric breaks down, and electrical shorts can occur at the location of the breakdown. This is a type of electrostatic discharge (ESD) damage. Shorts between the sensor and the shields are detrimental to the operation of the head. A typical specification on the resistance between the shields and the sensor is 100 kOhms. Accordingly, any head with a resistance less than 100 kOhms between the read sensor and either shield fails such a test. Losses at wafer final test due to shield shorts can be as high as 30%. One way to prevent the charging of the shields is to electrically short both shields to one side of the sensor via a lead and then remove the short during slider fabrication. While this will provide protection against process-induced charging, it does not allow the ability to test for shield shorts due to other phenomena, such as pinholes in the gap dielectric.